The present invention relates to solvent-based or solvent-free compositions which provide silicone pressure sensitive adhesives, hereinafter also referred to as SPSAs, which are stable in the absence of moisture and which cure by the action of ambient moisture. In a preferred embodiment the present invention relates to SPSA compositions which cure to a permanent adhesive which is useful as a structural adhesive.
By the term "silicone pressure sensitive adhesive" it is meant an adhesive comprising one or more siloxane components and possessing sufficient tack and adhesive strength so that it can be adhered with mild pressure to a clean stainless steel substrate and then removed therefrom and adhered to the same or another clean substrate. By the term "permanent adhesive" it is meant an adhesive that bonds to a clean substrate and cannot be so removed and thereafter readhered.
At the present time all commercial SPSA compositions are designed for the relatively low strength bonding of surfaces, such as electrical insulating tapes, medical tapes and devices and high temperature masking tapes. These commercial SPSA compositions are also designed to possess their ultimate adhesive properties when first applied.
However, there is a need for a SPSA which will provide higher bonding strengths and/or which will develop additional adhesive properties, such as resistance to flow or long term durability, after being applied.
Silicone structural glazing of curtainwall panels represents one of those needs. Durable structural glazing is currently done with silicone sealants in a multi-step, labor-intensive, time-consuming process. Significant improvements in the structural glazing process could be realized if a SPSA were used in place of the silicone sealant in the current process.
Unfortunately, standard SPSAs are not completely suitable for structural glazing applications because of their inherent flowability, albeit slow in the recently improved SPSAs, under sustained stress. Under the sustained stress of gravity the heavy glass panel that is currently used in structural glazing, if not further supported, would eventually experience shear failure of a SPSA bond. A SPSA that cures to a non-shearing adhesive having high adhesive strength would be very beneficial in this and other applications.
Currie et al., U.S. Pat. No. 2,814,601, disclose SPSA compositions which are useful for bonding silicone rubber to metal such as aluminum, and which will cure to a permanent bond adhesive at room temperature.
Blizzard et al., U.S. Pat. No. 4,613,534 claims a permanent-bond adhesive for bonding coated fabric that is used in the construction of air- or tension-supported structures.
However, the compositions of Currie et al. and of Blizzard et al. cure spontaneously at room temperature when mixed, even in the absence of moisture, and therefore cannot be mixed and stored before use but, rather, must be used shortly after being prepared.
In a co-pending application for U.S. patent Ser. No. 07/748,477 filed on even date herewith and assigned to the assignee of this application, there is disclosed and claimed a moisture-curable SPSA composition which is stable in the absence of moisture when a solvent is present. However, when solvent is removed, such as when the composition is applied to a substrate and devolatilized, the SPSA has one or more deficiencies, such as limited shelf life, low tack and low wettability and/or conformability on a substrate such as glass or metal.